1. Field of the Invention
The present invention relates to a method and system for the prevention of corrosion of conductive structures combining organic coatings and semiconductor technology.
2. Discussion of the Background Art
A variety of methods for controlling corrosion have evolved over the past several centuries, with particular emphasis on methods to extend the life of metallic structures in corrosive environments. These methods typically include protective coatings which are used principally to upgrade the corrosion resistance of ferrous metals, such as steel, and some nonferrous metals, such as aluminum, and to avoid the necessity for using more costly alloys. Thus, they both improve performance and reduce costs. However, such protective coatings typically have several pitfalls, including poor applicability to non-metallic structures that suffer from corrosion or fouling.
Protective coatings fall into two main categories. The largest of these categories is the topical coating such as a paint, that acts as a physical barrier against the environment. The second category consists of sacrificial coatings, such as zinc or cadmium, that are designed to preferentially corrode in order to save the base metal from attack.
Cathodic protection and coatings are both engineering disciplines with a primary purpose of mitigating and preventing corrosion. Each process is different: cathodic protection prevents corrosion by introducing an electrical current from external sources to counteract the normal electrical chemical corrosion reactions whereas coatings form a barrier to prevent the flow of corrosion current or electrons between the naturally occurring anodes and cathodes or within galvanic couples. Each of these processes provided limited success. Coatings by far represent the most wide-spread method of general corrosion prevention (see Leon et al U.S. Pat. No. 3,562,124 and Hayashi et al U.S. Pat. No. 4,219,358). Cathodic protection, however, has been used to protect hundreds of thousands of miles of pipe and acres of steel surfaces subject to buried or immersion conditions.
The technique of cathodic protection is used to reduce the corrosion of the metal surface by providing it with enough cathodic current to make its anodic dissolution rate become negligible (for examples, see Pryor, U.S. Pat. No. 3,574,801; Wasson U.S. Pat. No. 3,864,234; Maes U.S. Pat. No. 4,381,981; Wilson et al U.S. Pat. No. 4,836,768; Webster U.S. Pat. No. 4,863,578; and Stewart et al U.S. Pat. No. 4,957,612). The cathodic protection concept operates by extinguishing the potential difference between the local anodic and cathodic surfaces through the application of sufficient current to polarize the cathodes to the potential of the anodes. In other words, the effect of applying cathodic currents is to reduce the area that continues to act as an anode, rather than reduce the rate of corrosion of such remaining anodes. Complete protection is achieved when all of the anodes have been extinguished. From an electrochemical standpoint, this indicates that sufficient electrons have been supplied to the metal to be protected, so that any tendency for the metal to ionize or go into solution has been neutralized.
Recent work in the study of corrosion has found that electrochemical corrosion processes appear to be associated with random fluctuations in the electrical properties of electrochemical systems, such as cell current and electrode potential. These random fluctuations are known in the art as xe2x80x9cnoisexe2x80x9d. Researchers have begun to apply noise analysis techniques to study the processes of corrosion in electrochemical systems.
Riffe, U.S. Pat. No. 5,352,342 and Riffe U.S. Pat. No. 5,009,757 disclose a zinc/zinc oxide based silicate coating that is used in combination with electronics in a corrosion prevention system. The zinc/zinc oxide particles in the coating are disclosed as having semiconductor properties, primarily a p-n junction at the Znxe2x80x94ZnO phase boundary. When reverse biased, this p-n junction is described as behaving as a diode and inhibiting electron transfer across the boundary. This restriction limits electron transfer from sites of Zn oxidation to the sites of oxygen reduction on the ZnO surface. Effectively, there is increased resistance between the anode and cathode of local corrosion cells and corrosion is reduced.
On average, the Znxe2x80x94ZnO based junction will be reversely biased due to the potentials associated with the oxidation of Zn at the Zn surface and the reduction of O2 at the ZnO surface. However, significant stochastic voltage fluctuations occur. These voltage fluctuations cause the junction to episodically become forward biased. When forward biased, electron transfer across the junction increases and there is an acceleration of the oxidation of Zn and reduction of O2. Effectively, there is a short circuit between the anode and cathode of local corrosion cells and corrosion is enhanced.
The Riffe patents disclose attachment of a fixed value capacitor in the electrochemical circuit of the corrosion prevention system. However, there is no way to control the level of capacitance nor any method suggested for determining the level of capacitance needed to effectively prevent corrosion in any given structure. Hence, it is necessary to use an overcapacitance in the system to be effective.
Recently, the development of conductive organic polymers has reached the point where they are commercially feasible. Their uses range from charge-storage batteries, antistatic films, conductive hosings, gaskets, cable shields, conductive textiles, chemical sensors, electromagnetic shielding, gas separation membranes, electrooptic devices, discharge layers in electrolithographic applications, and as corrosion-preventive paints. One such corrosion preventive application is a commercial product known as CATIZE, available from GeoTech Chemical Company, LLC through its distributor Seegott, Inc. of Ohio. This is a conductive polyaniline polymer doped with zinc metal or ions, which is used as a sacrificial cathodic protective layer on metal structures.
One drawback to previous corrosion preventive methods, such as that of Riffe disclosed above, is the relative inflexibility of color selection available for the silicate based coatings disclosed therein, with the only color readily available being grey. While this is acceptable in most marine and structural uses, there is a need for corrosion preventive coatings that are non-sacrificial and which can be provided in a range of colors for use as paint substitutes, particularly in the automotive and transportation industries.
Accordingly, one object of the present invention is to provide an organic conductive polymer coating that provides anticorrosion properties to any conductive structure.
A further object of the present invention is to provide a method for protecting conductive metallic structures from corrosion that is fine-tuned to the unique characteristics of the metallic structure.
A further object of the present invention is to provide a method for preventing corrosion of conductive structures by using organic polymer based semiconductor technology and with no external anode, no electrolyte, and no current flow.
A further object of the present invention is to provide a system for protecting conductive structures from corrosion, wherein the system provides long term protection with minimal system maintenance required.
A further object of the present invention is to provide an organic polymer coating having anti-corrosion properties and which can be provided in any desired color for use as a paint substitute.
These and other objects have been satisfied by the discovery of a semiconductive organic polymer coating and associated electronic system, wherein the system can be operated by merely filtering voltage fluctuations in the conductive structure on which the semiconductive organic coating is placed, wherein the method for using the system comprises:
coating the conductive structure with a semiconductive organic polymer coating with a fixed electronic filter connected to said coated structure,
monitoring noise generated by said coating having said fixed electronic filter connected thereto,
using an adjustable filter connected to said coating to determine an anti-corrosive filter response needed to minimize the noise generated by said coating; and
replacing said adjustable filter with a passive or active filter having a filter response of at least said anti-corrosive filter response.